Device for crucible-free, floating-zonemelting a crystalline member

ABSTRACT

Device for zone melting a crystalline member includes a vessel, an electrically energizable heating device mounted in the vessel for heating a melting zone formed in a crystalline member, and an electric current supply for the heating device extending into the vessel through an opening formed in a wall of the vessel, the supply comprising at least two conductors insulated from one another by an intermediate layer of insulating material having a Shore hardness in the range of 10 to 100 and an extensibility of at least 20 percent, a gas-tight adhesive connection being provided between the intermediate layer of insulating material and the surface of the conductors.

[ 1 Aug. 29, 1972 United States Patent Keller 2,617,002 11/1952Hasley...................174/152X 2,756,375 7/1956 Peck ...174/l52UX[54] DEVICE FOR CRUCIBLE-FREE,

FLOATING-ZONEMELTING A CRYSTALLINE MEMBER Primary ExaminerRoy N. Envall,Jr.

[72] Inventor:

g Keller Pretzfeld AttorneyCurt M. Avery, Arthur E. Wilfond, Herbert L.Lerner and Daniel J. Tick [73] Assignee: Siemens Aktiengesellschaft,Berlin and Munich, Germany ABSTRACT March 1970 Device for zone melting acrystalline member includes [21] APPLNOJ 23,357 a vessel, anelectrically energizable heating device mounted in the vessel forheating a melting zone [22] Filed:

formed in a crystalline member, and an electric cur- [30] ForeignApplication Prlorlty Data March 29, 1969 Germany.........

rent supply for the heating device extending into the p 19 1 31 9 vesselthrough an opening formed in a wall of the vessel, the supply comprisingat least two conductors in- 174/152 R, sulated from one another by anintermediate layer of insulating material having a Shore hardness in the[52] US. Cl...................13/l, l3/DIG. 1

219/ 10.79 [51] Int. 11/06, HOSb 5/08 range of 10 to 100 and anextensibility of at least 20 [58] Field of Search..........13/DIG. 1,1,20; 174/151, percent, a g g adhesive connection be g pro- 174/ 152 R;219/ 10.79 vided between the intermediate layer of insulating materialand the surface of the conductors.

3 Claims, 3 Drawing Figures [56] References Cited UNITED STATES PATENTS2,904,663 9/1959 Emeis et al.............13/12IG. 1

PATENTED M1829 I972 DEVICE FOR CRUCIBLE-FREE, FLOATING- ZONEMELTING ACRYSTALLINE MEMBER My invention relates to device for zone melting acrystalline member.

A device foe zone melting, especially for cruciblefree, floating zonemelting of a crystalline member, generally includes a vessel in whichholder for the crystalline member and a heating device for heating amolten zone in the crystalline member are disposed. The vessel is eitherevacuated or is filled with protective gas such as noble gases or amixture of hydrogen and nitrogen, for example. The heating device isenergized with electric current and consequently is formed of a currentsupply made up of at least two conductors and which passes through anopening in the vessel wall from the outside. The current supply can be,for example, a coaxial conductor assembly wherein the intermediate spacebetween both tubular conductors is filled with an insulating material.This intermediate layer of insulating material such as band-formingpolytetrafluorethylene cast in paraffin or the material known under thetrade name Araldit has been found to be virtually gas-tight whenemployed in a zone melting process of semiconductor rods which are nottoo thick.

Especially when zone melting, for example, relatively thicksemiconductor rods for which a relatively high power is required andwherein therefore a relatively high power loss occurs in the currentsupply, it was observed that undesired oxide layers always formed on thesemiconductor rods being treated in spite of careful sealing of themelting chamber. The invention of the instant application is based uponthe realization that the cause of this are fine leaks which form betweenthe intermediate layer of insulating material and the surface of theconductors of the current supply during the zone melting operationbecause the insulating material tends to tear at least partly due tothermal expansion and contraction of the conductors. Due to these leaks,the outer atmosphere can penetrate into the melting chamber and producethe oxide coatings.

It is accordingly an object of the invention to provide device foecrucible-free, floating zone melting a crystalline member which avoidsthe foregoing disadvantage of the heretofore known devices of thisgeneral type.

More specifically it is an object of the invention to provide suchdevice which avoids the formation of leaks and consequently preventspenetration of the surrounding atmosphere into the melting chamber andthe production of oxide layers.

With the foregoing and other objects in view I provide, in accordancewith my invention, a device for zone melting a crystalline membercomprising a vessel, an electrically energizable heating device mountedin the vessel for heating a melting zone formed in a crystalline member,and an electric current supply for the heating device extending into thevessel through an opening formed in a wall of the vessel, the supplycomprising at least two conductors insulated from one another by anintermediate layer of insulating material having a Shore hardness in therange of to 100 and an extensibility of at least 20 percent, a gas-tightadhesive connection being provided between the intermediate layer ofinsulating material and the surface of the conductors.

If the current supply is nondisplaceably disposed in the openingextending through the vessel wall, in accordance with another feature ofthe invention, I fill that opening with insulating material which has aShore hardness within the limits of 10-100 and extensibility of at least20 percent, and a gas-tight adhesive connection is provided between theinsulating material, on the one hand, and the vessel wall as well as thesurface of at least one of the conductors, on the other hand.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin device for crucible-free, floating zone melting a crystalline member,it is nevertheless not intended to be limited to the details shown,since various modifications and structural changes may be made thereinwithout departing from the spirit of the invention and within the scopeand range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawing, in which:

FIG. 1 is a sectional view of part of a wall of a vessel for zonemelting a semiconductor rod through an opening of which there extends acurrent supply formed of two conductor tubes disposed coaxially to oneanother;

FIG. 2 is a view similar to that of FIG. 1 wherein instead of thecoaxial conductor tubes of FIG. 1, a current supply formed of twowire-shaped current conductors extends through an opening in the vesselwall; and

FIG. 3 is a diagrammatic view of a complete zone melting apparatusaccording to the invention.

Referring now to the drawing and first particularly to FIG. 1 thereof,there is shown a fragmentary section of a wall 21 of a vessel formingpart of a device for zone melting a crystalline member. An opening orpassage 21a is provided in the wall portion 21. Within the passage 210there is disposed a current supply formed of two conductor tubes 26 and27 of copper which are located coaxial to one another. At the upper endA of the conductors 26 and 27 there is connected a current source 40(FIG. 3), for example, a high frequency generator, while the lower end Bwhich is within the vessel is connected to a heating device 41 forheating a molten zone 42 formed in a crystalline rod 43 which isend-supported in the vessel. As shown, the heating device 41 is in theform of an induction heafing coil.

Both the coaxial conductor tubes 26 and 27 are of different diametersand are insulated from one another by a layer 28 of cold-vulcanizedsilicon rubber which is disposed intermediate The conductor tubes 26 and27. A good adhesive connection between the insulating intermediate layer28 and the conductor tubes 26 and 27 is ensured by means of siliconresin coating 30 applied to the inner surface of the outer conductortube 26 and a silicon resin coating 31 applied to the outer surface ofthe inner conductor tube 27 The inner conductor tube 27 can also have asilicon resin coating 32 applied to the inner surface thereof and can begas-tightly closed by means of a stopper 29 formed of cold-vulcanizedsilicon rubber.

Cold-vulcanized silicon rubbers which are suitable for theaforementioned purposes are, for example those of the West German firmWacker having the trade names Sil-Gel and Vergussmasse K. The siliconresin coatings on the surfaces of the conductor tubes 26 and 27 can beformed of the silicon resin produced by the same firm Wacker under thetrade name G 718.

Cold-vulcanized silicon rubber is of Shore hardness in the range of -100and an extensibility of percent. A cold-vulcanized silicon rubber havinga Shore hardness of 50 and an extensibility of about 100 percent ispreferred. Cold-vulcanized silicon rubber can therefore readily expandat temperature variations without being torn away from the silicon resincoatings 30, 31, and 32 and thereby from the surfaces of the conductortubes 26 and 27. Shore hardness is a measure of hardness which isdetermined by a balldropping test (recoil or rebound measuring); noteR'o'mpp Chemie bexikon, Vol. 6, Frankische Verlagshandlung, Stuttgart,page 5,874.

A further advantage of an intermediate layer of insulating materialconsisting of cold-vulcanized silicon rubber is that the dielectric lossfactor thereof is smaller than 10 and the dielectric strength thereoflies between 30 and 40 kV/mm so that this intermediate layer is alsowell suited for a current supply through which a high frequencyinduction heating coil is provided with electric energy.

The opening 21a is also filled with cold-vulcanized silicon rubber 24.The surfaces of the vessel wall 21 and of the outer conductor tube 26are provided within the opening 21a with silicon resin coating 23 and towhich the silicon rubber 24 firmly adheres so that the opening 21a isgas-tightly closed.

The current supply formed of the coaxial tubes 26 and 27 can also bedisplaceable in axial direction in the passage or opening 21a formed inthe vessel wall 21. In such a case, instead of the silicon resincoatings 23 and 25 as well as the silicon rubber filling 24, and oilseal can be located, for example, within the opening 21a to effectsealing thereof. Also in such a case it is advantageous moreover for thecoaxially disposed conductor tubes 26 and 27, exactly as shown in FIG.1, to be insulated from one another by an intermediate layer ofcold-vulcanized silicon rubber and for the inner tube 27 to begas-tightly closed with stopper 28 of cold-vulcanized silicon rubber.

Advantageously, my invention is also utilizable for a passage throughthe wall of a vessel employed in a zone melting operation wherein acurrent supply formed of two wireshaped conductors is disposed. In FIG.2, such an opening or passage 11a in a section 11 of the wall of avessel forming part of a device for zone melting of a crystalline memberis illustrated. The current supply is formed of conductors 12 consistingof two copper wires. At the upper end A of the copper wires 12, a

nonillustrated current source such as a high frequency generator isconnected, while the lower end B thereof is connected with anonillustrated heating device for heating the molten zone formed in acrystalline member that is mounted in the melting zone vessel. Withinthe opening or passage 11a, the vessel wall 11 and the surface of thecopper wires 12 are provided with coatings 13 and 15 formed of siliconresin.

hesive connection exists between the silicon rubber 14 and the siliconresin coatings 13 and 15.

The mounting of current supplies according to FIGS. 1 and 2 into theopenings or passages formed in the vessel wall is especially simple whenemploying cold-vulcanized silicon rubber. This is because, after theapplication of silicon resin coatings on the conductors of the currentsupply and on the vessel wall surface within the opening or openings aswell as within the inner spaces of the conductor tubes 26 and 27 in FIG;1, molding can be readily effected with the initially liquid siliconrubber to which a hardener such as, for example, a hardener 'T of theaforementioned firm Wacker has been added.

I claim:

1. Device for zone melting a crystalline member comprising a vessel, anelectrically energizable heating device mounted in said vessel forheating a melting zone formed in a crystalline member and an electriccurrent supply for said heating device extending into said vesselthrough an opening formed in a wall of said vessel, said supplycomprising at least two conductors insulated from one another by anintermediate layer of insulating material having a Shore hardness in therange of 10 to 100 and an extensibility of at least 20 percent, agas-tight adhesive connection being provided between said intermediatelayer of insulating material and the surface of the conductors, theopening formed in said vessel wall being filled with insulating materialalso having a Shore hardness in the range of 10 to and an extensibilityof at least 20 percent, and a gas-tight adhesive connection beingprovided between the insulating material, on the one hand, and saidvessel wall as well as the surface of at least one of said conductors,on the other hand.

2. Device according to claim I, wherein said insulating material iscold-vulcanized silicon rubber, and the surface of said one conductorhas a layer of silicon resin at the location thereof at which it isadhesively connected to the silicon rubber insulating material, saidsilicon rubber insulating material adhering to said layer of siliconresin.

3. Device according to claim 2, wherein said silicon rubber has aShore-hardness of substantially 50 and an extensibility of substantially100 percent.

2. Device according to claim 1, wherein said insulating material iscold-vulcanized silicon rubber, and the surface of said one conductorhas a layer of silicon resin at the location thereof at which it isadhesively connected to the silicon rubber insulating material, saidsilicon rubber insulating material adhering to said layer of siliconresin.
 3. Device according to claim 2, wherein said silicon rubber has aShore-hardness of substantially 50 and an extensibility of substantially100 percent.